For recent window glass for automobiles, curved glass having various shapes and curvatures is required in complying with a change in design. As a technique to produce such curved glass, there is a press forming method to be conducted in a heating furnace. This method is suitable as a method for producing curved glass having a complicated shape or a deep-set shape because a glass sheet can be bent under elevated temperature.
Heretofore, in the press forming method for bending a glass sheet in a heating furnace, there has been known a forming apparatus wherein a support frame is used as a lower mold on which the glass sheet is placed; in preforming the glass sheet before press-forming, the glass sheet is deformed due to its own weight and then, the glass sheet is pressed against a forming surface of forming mold.
In this bend forming apparatus, the glass sheet heated to nearly its softening point while it is conveyed in the heating furnace by means of rollers, is placed on the support frame by means of a transfer machine having a positioning mechanism. The support frame holding the glass sheet moves to a downstream pressing section where the glass sheet is pressed between an upper forming mold and the lower support frame to be bent in a predetermined curved shape.
In this case, it is necessary to determine the position between the upper forming mold and the lower support frame with high precision in the pressing section. Heretofore, as the method for positioning (aligning) the upper forming mold and the lower support frame, there has been known such a technique that positioning pins are formed in the upper forming mold so as to extend to the support frame, sockets are provided in the lower support frame, and the positioning of the forming mold to the support frame is performed by fitting the positioning pins into the sockets.
As another method, there is proposed such a method as described in, for Example, JP-A-6-247728. According to this method, a sensor is provided on a forming mold, an object to be detected by this sensor is provided on a support frame, a displacement of position between the forming mold and the support frame at the time of positioning is detected by the detecting means comprising said sensor and said object to be detected, and the support frame is moved based on the displacement detected, whereby the position determination is conducted with high precision.